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Building Notification Payloads

Delivering push notification through APNS requires three things: your SSL certificate, a device ID, and a custom payload with the notification you want to send. The payload uses JSON formatting. You've already read about generating the certificate and producing the device identifiers, which you need to pass up to your server. Building the JSON payloads basically involves transforming a small well-defined dictionary into JSON format.

JSON (JavaScript Object Notation) is a simple data interchange format based on key-value pairs. The JSON Web site (www.json.org) offers a full syntax breakdown of the format, which allows you to represent values that are strings, numbers, and arrays. The APNS payload consists of up to 256 bytes, which must contain your complete notification information.

Notification payloads must include an aps dictionary. This dictionary defines the properties that produce the sound, badge, and/or alert sent to the user. In addition, you may add custom dictionaries with any data you need to send to your application so long as you stay within the 256 byte limit. Figure 16-11 shows the hierarchy for basic (nonlocalized) alerts.

Figure 16-11. The aps dictionary may contain one or more notification types including a badge request, a sound file, and/or an alert.

aps
     badge : number
     sound : sound file name string
     alert : string
     alert
           body : string
           action-loc-key : string

The aps dictionary contains one or more notification types. These include the standard types you've already read about: badges, sounds, and alerts. Badge and sound notifications each take one argument. The badge is set by a number, the sound by a string that refers to a file already inside the application bundle. If that file is not found (or the developer passes default as the argument), a default sound plays for any notification with a sound request. When a badge request is not included, the iPhone removes any existing badge from the application icon.

There are two ways to produce an alert. You can pass a string, which defines the message to show. This automatically produces a notification with two buttons under that message: Close and View. To customize buttons, pass a dictionary instead. Send the message text as the body and the string to use for the Action key (normally View) as action-loc-key. This replaces View with whatever text you specify.

To produce an alert with a single OK button, pass null as the argument to action-loc-key. This creates a special alert style with one button. Just as when a user taps Close, the OK style alert will not pass any data directly to your application. The app must poll for any updates when next opened by the user.

Localized Alerts

When working with localized applications, construct your aps > alert dictionary with two additional keys. Use loc-key to pass a key that is defined in your application's Localizable.strings file. The iPhone looks up the key and replaces it with the string found for the current localization.

At times, localization strings use arguments like %@ and %n$@. Should that hold true for the localization you are using, you can pass those arguments as an array of strings via loc-args. As a rule, Apple recommends against using complicated localizations as they can consume a major portion of your 256-byte bandwidth.

Transforming from Dictionary to JSON

Once you've designed your dictionary, you must transform it to JSON. The JSON format is simple but precise. If you can, use an automated library to convert your dictionary to the JSON string. There are numerous solutions for this for any number of programming languages, including JavaScript, Perl, and so on. Here's a quick rundown of JSON basics. Table 16-1 offers examples of these rules in action.

Table 16-1. JSON Payload Samples

Sample Type

JSON

Hello message, displays with two buttons.

{"aps":{"alert":"hello"}}

Hello message, displays with two buttons, but built using JSON with an alert dictionary.

{"aps":{"alert":{"body":"hello"}}}

Hello message with one OK button.

{"aps":{"alert":{"action-loc-key":null,"body":"hello"}}}

Hello message with two buttons, Close and Open, the latter being a custom replacement for View.

{"aps":{"alert":{"action-loc-key":"Open","body":"hello"}}}

Hello message that adds an application badge of 3.

{"aps":{"badge":3,"alert":{"body":"hello"}}}

Play a sound without an alert.

{"aps":{"sound":"ping2.caf","alert":{}}}

Play sound, display badge, display alert, use a custom button.

{"aps":{"sound":"ping2.caf","badge":2,"alert":{"action-loc-key":"Open","body":"Hello"}}}

Add a custom payload including an array.

{"aps":{"alert":{"body":"Hello"}},"key1":"value1","key2":["a","b","c"]}

  • The entire payload is a dictionary. Dictionaries consist of key-value pairs stored between brackets, that is, {key:value, key:value, key:value, ...}.
  • Key-value pairs are separated with commas.
  • Strings use double quotes; numbers do not. Reserved words include true, false, and null. Reserved words are not quoted.
  • Arrays consist of a list of items between square brackets, that is, [item, item, item,...].
  • The following symbols must be escaped in strings by using a backslash literal indicator: '" \ /.
  • You may want to remove carriage returns (\r) and new lines (\n) from your payloads when sending messages.
  • Spaces are optional. Save space by omitting them between items.
  • The aps dictionary appears within the top-level folder, so the most basic payload looks something like {aps:{}}.

Custom Data

So long as your payload has room left, keeping in mind your tight byte budget, you can send additional information in the form of key-value pairs. As Table 16-1 showed, these custom items can include arrays and dictionaries as well as strings, numbers, and constants. You define how to use and interpret this additional information. The entire payload dictionary is sent to your application so whatever information you pass along will be available to the application: didReceiveRemoteNotification: method via the user dictionary.

A dictionary containing custom key-value pairs does not need to provide an alert, although doing so allows your user to choose to open your application if it isn't running. If your application is already launched, the key-value pairs arrive as a part of the payload dictionary.

Receiving Data on Launch

When your client receives a notification, tapping the action key (by default, View) launches your application. Then after launching, the iPhone sends your application delegate an optional callback. The delegate recovers its notification dictionary by implementing a method named application:didFinishLaunchingWithOptions:. Unfortunately, this method might not work properly. So here are both the standard ways of retrieving notification information plus a work-around.

Normally, the iPhone passes the notification dictionary to the delegate method via the launch options parameter. For remote notifications, this is the official callback to retrieve data from an alert-box launch. The didReceiveRemoteNotification: method is not called when the iPhone receives a notification and the application is not running.

This "finished launching" method is actually designed to handle two completely different circumstances. First, it handles these notification alert launches, allowing you to recover the payload dictionary and use the data that was sent. Second, it works with application launches from openURL:. If your app has published a URL scheme, and that scheme is used by another application, the application delegate handles that launch with this method.

In either case, the method must return a Boolean value. As a rule, return YES if you were able to process the request or NO if you were not. This value is actually ignored in the case of remote notification launches, but you must still return a value.

At the time of writing, implementing this method does not work properly. The application will hang without displaying a GUI. Fortunately, there's an easy work-around that does not rely on the callback method. You can, instead, listen for a launch notification and catch the userInfo dictionary that is sent with it. This solution has the advantage of being reliable and tested. Keep an eye on Apple's developer forums (http://devforums.apple.com) to keep track of when this issue gets fixed.

Start by adding your application delegate as a listener via the default NSNotificationCenter in your normal applicationDidFinishLaunching method.

[[NSNotificationCenter defaultCenter] addObserver:self
    selector:@selector(launchNotification)
    name:@"UIApplicationDidFinishLaunchingNotification" object:nil];

Then implement the method for the selector you provided. Here, the application waits for the GUI to finish loading and then displays the user info dictionary, where the remote notification data has been stored.

- (void) launchNotification: (NSNotification *) notification
{
    [self performSelector:@selector(showString) withObject:
    [[notification userInfo] description] afterDelay:1.0f];
}

Between the notification listener and the method callback, you can reliably grab the user data from remote notifications. This work-around should remain viable regardless of when and how Apple addresses the didFinishLaunchingWithOptions method.

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